You could drop matter in there and get 40% of the mass back as energy. Not that it's useful energy for us at the moment, but a more advanced civilization could probably learn how to ensure most comes out as polar jets and harvest that somehow.

https://web.stanford.edu/~wilkinsd/docs/posters/BlackHolesPo...

Easier to build a Dyson sphere around it than the sun.

A Dyson sphere around a body with 4.5cm radius sounds quite manageable. I wonder how small a Dyson sphere could be with current materials. I imagine it's mostly limited by gravitational forces at that point?

or some sort of roman war bird.

> roman war bird

Roman? Did you maybe mean Romulan?

I think you can also extract energy from it by slingshoting an object off the black hole to draw energy from the black hole's spin.

Would it be useful to have a large power plant at the edge of the solar system? There's not much out there to power.

Microwave transmission could send it somewhere else... wirelessly: https://en.wikipedia.org/wiki/Wireless_power_transfer#Microw...

You could use it to execute prisoners perhaps. Death by blackhole

The CO2 cost of getting the prisoners from the prison outside earth's gravity well and on their way to planet 9 seems prohibitive.

One more reason for rehabilitation as prison policy.

you'd have to space bus the prisoners, a few hundred thousand at a time. during the journey they might mutiny, take over the ship and set a course for a nearby livable planet.

One of aphorisms of Kozma Prutkov[1] was this: "Throwing pebbles into the water, look at the ripples they form on the surface. Otherwise this activity will be an empty amusement."

I believe, that throwing pebbles into black hole, and watching the ripples they form would be very amusing.

[1] https://en.wikipedia.org/wiki/Kozma_Prutkov

It would be a cheap source of additional speed for spacecraft in the outer solar system, by means of powered gravity assists. See https://en.wikipedia.org/wiki/Oberth_effect

> additional speed for spacecraft

Extreme rip-your-craft-to-bits tidal effects [0] would make life exciting. Also, see 'Neutron Star' by Larry Niven for a description of such a manoeuvre, albeit a less dangerous one than than sling-shoting a black Hole because of the larger size of the neutron star.

[0] https://en.wikipedia.org/wiki/Tidal_force

If you were really precise, you could tune your assist based on distance. You could get a pretty sick change in angular momentum relative to the sun since you wouldn't have to worry about hitting atmosphere or terrain. If you're far away you feel nothing, if you're too close you join the singularity, and everything in between is gravy.

Imagine getting a 1970's style planetary alignment, going with a Voyager style slingshot and your final assist is a couple thousand miles away from a black hole.

If it can cause some good amount of time dilation, then send people into future by having them orbit around it.

It might be better to wait

Maybe more easily study the effects of relativity by sending a spacecraft to it?

Sending a spacecraft out hundreds of AU is doable with current technology, but it would take many decades.

Depending on the size of it, harvest energy from its Hawking radiation.

As stated in the paper, the amount of Hawking radiation coming off it is absolutely miniscule. The black-body temperature of the emissions is much much lower than the temperature if you just look into space.

See also https://en.wikipedia.org/wiki/Penrose_process

That doesn’t look like a lot of energy, but we could enclose it in a dishwasher sized box, which is kind of a funny concept.

As for the size, the paper has a 1:1 scale drawing of it.

It would have to be a pretty sturdy box. Even if such a box could be built, it would be tricky to keep the black hole in the exact center.

I would like to upvote this 1000 times to underline "pretty sturdy box." It can be a particularly amusing exercise to do the math for things like this, calculate the gravitational forces involved, and compare the results to e.g. the strength of steel.

The net gravitational force is zero: https://en.wikipedia.org/wiki/Shell_theorem

I don't see how Shell theory applies here. The box wouldn't be inside the blackhole, it'd be outside it.

If the box were a perfect sphere then yes it would have zero net gravitational pull on the black hole, but the black hole would still have an extremely strong pull on it. And that's still assuming it's perfectly centered which would be almost impossible to maintain.

> but the black hole would still have an extremely strong pull on it

No, it would not. Newton's third law: if an object has zero net pull on the black hole, the black hole also has to have a zero net pull on the object. The shell theorem is more general than you are assuming: it doesn’t matter if the heavier object is inside or outside, the surrounding object doesn't need to be spherical (it can be any shape as long as it fully envelopes the black hole) and the black hole doesn't need to be centered. I know it is a counter-intuitive result, but it can be easily proven with calculus.

See for instance this sentence about Dyson shells: "Such a shell would have no net gravitational interaction with its englobed star (see shell theorem)" https://en.wikipedia.org/wiki/Dyson_sphere#Dyson_shell

Edit: Related, what is bigger? The gravitational force the Moon exerts on Earth or the gravitational force the Earth exerts on the Moon? Veritasium has a nice video explaining the answer: https://www.youtube.com/watch?v=8bTdMmNZm2M

The net pull on the _center of mass_ of a shell would be zero. The pull on any local region would still exist though. All that the shell theorem says is that the sum of all those local directional forces is zero.

Reading the exact passage you quoted, "the compressive strength of the material forming the sphere would have to be immense to prevent implosion due to the star's gravity."

You're wrong despite your confidence. Cage will have zero net gravitational effect on the black hole (assuming the cage is perfectly symmetrical), but that just means it won't by itself exert gravitational force on the black hole (and vice versa) causing it to move in any direction.

Cage's material will still get attracted to the black hole and close to the event horizon structural forces exerted by gravity will be hard / impossible to counteract with current materials: https://en.wikipedia.org/wiki/Strength_of_materials

I also exert zero net pull when I tear open a package of snacks. Yet the package is destroyed in the process.

You'd probably want to use active control systems to move the box around to follow the black hole, with rockets when necessary. However, by lanna's reasoning below from the shell theorem, if the box were a spherical shell, it wouldn't be an unstable equilibrium like a ringworld, so your active control system could be pretty sluggish.

Such a box can definitely be built if you make it much larger than a dishwasher.

A box made of any known material that close in would get ripped to shreds by tidal forces.

I was under the impression that from 50 cm (middle of the sides) to about 70 cm (the corners of the dishwasher) it wouldn't be that bad, but I'll need to do the math.

Finally solves the problem of what to do with cable news pundits.